Wireless communication systems are widely used to provide voice and data services for multiple users using a variety of access terminals such as cellular telephones, laptop computers and various multimedia devices. Such communications systems can encompass local area networks, such as IEEE 801.11 networks, cellular telephone and/or mobile broadband networks. The communication system can use one or more multiple access techniques, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA) and others. Mobile broadband networks can conform to a number of standards such as the main 2nd-Generation (2G) technology Global System for Mobile Communications (GSM), the main 3rd-Generation (3G) technology Universal Mobile Telecommunications System (UMTS) and the main 4th-Generation (4G) technology Long Term Evolution (LTE).
A wireless network may include a wireless device and a plurality of base stations. The wireless device may be a notebook computer, a mobile phone or a Personal Digital Assistant (PDA), a media player, a gaming device or the like. The base stations communicate with the wireless device over a plurality of wireless channels coupled between the wireless device and the base stations (e.g., a downlink channel from a base station to a wireless device). The wireless device may send back information, including channel information, to the base stations over a plurality of feedback channels (e.g., an uplink channel from the wireless device to the base station).
The wireless device may include a processor, a transmitter and a receiver. The transmitter may be coupled to one transmit antenna. The receiver may be coupled to a receive antenna. One major function of the receiver is rejecting unwanted noise such as adjacent channels and interference so that a desired signal from a wide spectrum of signals from the receive antenna can be recovered.
As wireless techniques further advance, direct-conversion receivers have emerged as an alternative and have been widely used in mobile phones. One advantageous feature of direct-conversion receivers is that the direct conversion receivers may not comprise bulky components such as intermediate frequency surface acoustic wave filters, intermediate frequency synthesizers and/or the like.
One disadvantageous feature of the direct conversion receivers is that even-order distortion such as the second-order intermodulation (IM2) may occur in the direct-conversion receiver. Especially, the down-conversion mixer of a direct-conversion receiver is a major source of the IM2 components. The IM2 components may comprise two parts, namely a differential mode IM2 component and a common mode IM2 component. Both the differential mode IM2 component and the common mode IM2 component are unwanted signals for mobile handsets and may cause interference and lead to deterioration in receiver performance.